1. Field Of The Invention
This invention relates to dielectric resonator oscillator and filter circuits and the like, and more particularly relates to a dielectric resonator oscillator circuit in which the intrinsic Q value of the dielectric resonator is significantly maintained after integration into an electrical circuit.
2. Description Of The Prior Art
Dielectric resonator oscillators are a class of stable microwave oscillators that are frequently used in radar and communication systems. Dielectric resonators are often utilized in oscillator circuits because of an intrinsically high Q value and excellent stability over broad temperature ranges. These characteristics allow an oscillator employing a dielectric resonator to have excellent frequency stability with only a small amount of phase noise over a wide range of environmental conditions. The Q value is defined as the ratio between the energy stored per cycle to the energy dissipated per cycle.
Dielectric resonators are usually made of a ceramic type material having a high dielectric constant (.epsilon.=30 to 40) and a low dissipative loss. These characteristics allow the dielectric resonator to store energy with relatively little internal energy dissipation. This corresponds to a high Q value for the dielectric resonator.
Every dielectric resonator has an intrinsic Q value which is its maximum Q value. However, this intrinsic Q value may decrease depending upon how the dielectric resonator is integrated in an electrical circuit. If the value of Q remains high even after integration into an electrical circuit, the dielectric resonator will have a significant frequency stabilization effect on the performance of the circuit. As the Q value decreases, the frequency stabilization effect will also decrease. It is known that oscillator output phase noise has the characteristic of changing in the relation 1/Q2. In other words, as the value of Q decreases the amount of associated output noise increases. In the case of a receiver local oscillator, the increased output noise can degrade the sensitivity of the receiver. When a dielectric resonator is employed in an oscillator circuit, it is imperative to integrate the dielectric resonator in a manner that does not significantly degrade the intrinsic Q value of the dielectric resonator so that a signal with low output noise can be generated.
Various printed transmission line mediums have been utilized with dielectric resonators in oscillator applications where a high degree of frequency stability (i.e. a high Q value) is required. In order to achieve the amount of magnetic field coupling required for oscillator performance, the dielectric resonator must be in relative close proximity to a transmission line of the oscillator circuit. FIG. 1 illustrates the traditional positioning of a dielectric resonator 100 and a printed microstrip transmission line 110. It is known to place the microstrip-dielectric resonator assembly in a metal enclosure 120 in order to reduce radiation losses and to thereby prevent an associated decrease in resonator Q value. In order to achieve sufficient magnetic field coupling, the dielectric resonator 100 should be positioned in relative close proximity to the microstrip transmission line 110. However, positioning the dielectric resonator 100 close to the microstrip transmission line 110 requires the resonator to be close to the ground plane 130 which causes the Q value of the resonator to significantly decrease.
Microstrip has traditionally been the most commonly used printed transmission line medium in dielectric resonator oscillators. However, microstrip and other commonly used transmission line mediums such as sandwich line, microguide, coplanar line and slot line have characteristics that cause degradation of the dielectric resonator Q value when used with a dielectric resonator. For example, when a dielectric resonator is used with microstrip, proper mutual magnetic field coupling is obtained when the resonator is in relative close proximity (approximately 0.01 to 0.02 inches) to the microstrip transmission line. But, since the dielectric resonator must be close to the microstrip transmission line, it will also be close to the ground plane and detrimental magnetic coupling between the dielectric resonator and the ground plane will result. The coupling of the dielectric resonator magnetic field with the ground plane causes dissipative loss from the dielectric resonator to the ground plane. This coupling significantly degrades the dielectric resonator Q value. It is known that the magnetic field coupling can cause a Q value reduction of approximately 50% for a dielectric resonator with an intrinsic Q value of 4500 at 22 GHz. As a result of the decrease in resonator Q value caused by the close proximity of the dielectric resonator to the transmission line ground plane, the full frequency stabilization effect of the dielectric resonator cannot be realized.
In an attempt to increase the Q value of the dielectric resonator, it is known to elevate the dielectric resonator above the microstrip substrate to maintain a sufficient distance between the dielectric resonator and the ground plane. FIG. 2 shows a dielectric resonator elevated above both a dielectric substrate 210 and microstrip transmission line 220 by a quartz spacer 230. This technique is commonly used to reduce dissipative losses due to the proximity of the ground plane and increase the Q value of the dielectric resonator 200. However, elevating the dielectric resonator a fixed distance above the dielectric substrate 210 and the microstrip transmission line 220 has the detrimental affect of decreasing the amount of magnetic field coupling between the resonator 200 and the microstrip transmission line 220. In order for an oscillator to operate efficiently, a significant amount of magnetic field line coupling is required.
If the dimensions of the metal enclosure (see FIG. 1) remain constant and the dielectric resonator is elevated above the microstrip substrate, the dielectric resonator will be closer to the top wall of the metal enclosure. The same effect is achieved if the top wall of the metal enclosure is brought closer to a stationary dielectric resonator. FIG. 3B illustrates what may occur to resonant frequency as the top wall of a metal enclosure 340, shown in FIG. 3A, is brought closer to the top of a stationary 22 GHz dielectric resonator. The resonator 300, shown in FIG. 3A, is located on top of a 0.10 inch quartz microstrip substrate 310 which has an integral ground plane 320. In addition, the quartz substrate is mounted on a 0.05 inch metal carrier 330 to provide rigidity. The dielectric resonator 300 is 0.12 inches in diameter and 0.048 inches in thickness. FIG. 3B shows an increasing resonator frequency as the top wall of the metal enclosure 340 is brought closer to the dielectric resonator 300. For example, when the top wall is 0.035 inches from the dielectric resonator, the resonator frequency is increased by 160 MHz as compared to when the top wall is more than 0.11 inches from the dielectric resonator. It is known that a reduction in distance from the substrate ground plane to the bottom of the resonator will have a similar but more significant affect on resonator frequency and resonator Q value.
A substrate cannot have unlimited thickness because the propagation characteristics of transmission lines are sensitive to the thickness of the substrate. In order to obtain the appropriate circuit impedance levels required for an oscillator circuit, typically at 22 GHz, a quartz substrate of 0.01 inches is used. However as previously stated, integration of a dielectric resonator with a microstrip line incurs an unavoidable decrease in Q value when the dielectric resonator is in close proximity to the substrate ground plane.
FIG. 4 shows an arrangement for the mechanical tuning of an H band dielectric resonator 400 utilizing a flat metal tuner plate 410. The thickness of the substrate 420 is 0.01 inches and its dielectric constant is 2.54. Movement of the flat metal tuner plate 410 has the same affect as displacing the entire top wall of the enclosure.
FIG. 5 illustrates the effect of displacing the flat metal tuner plate 410 shown in FIG. 4 in relation to the dielectric resonator 400. FIG. 5 shows how resonator frequency increases and resonator Q value decreases as the spacing between the top of the resonator and the tuner plate is reduced. For example, the Q value decreases from 8149 with a 6 mm (0.236 inch) space to a value of 3265 when there is almost no gap between the tuner plate 410 and the dielectric resonator 400. This reduction in resonator Q value corresponds to a decrease by a factor of approximately 2.5. FIG. 5 also illustrates a corresponding change in resonator frequency from 7.56 GHz to 9.08 GHz as the space between the tuner plate and the dielectric resonator is reduced from 6 mm to 0 mm. It is known that a similar relationship between resonator Q value and resonator frequency exists as the distance from the substrate ground plane to the bottom of the dielectric resonator decreases. In other words, the resonator frequency and resonator Q value may be dependent upon the thickness of the substrate.
Utilizing the configuration of FIG. 4, it can be inferred from FIG. 5 that a ground plane to resonator spacing of 0.01 inches can produce a significant decrease in resonator Q (factor of 2) and an increase (9.1%) in resonator frequency. This represents significant unwanted degradation of the resonator Q value.
It should be noted that there are also electronic filters that utilize dielectric resonators coupled to microstrips which undergo a significant degradation in resonator Q value due to the close proximity of the dielectric resonator to the transmission line ground plane.